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<br />. <br /> <br />55 <br /> <br />pervious foundations to be safe against uplift pressures and piping. Lane's <br /> <br />. <br /> <br />theory defined weighted-creep ratio as <br /> <br />c = <br />w <br /> <br />LLH + 3LLv <br />3H <br /> <br />(3-11) <br /> <br />. <br /> <br />where, <br /> <br />C = weighted-creep ratio, <br />w <br />LH = horizontal or flat contact distance (flatter than 450), <br /> <br />L = vertical or steep contact distance (steeper than 450), <br />v <br /> <br />. <br /> <br />H <br /> <br />head on structure (headwater - tailwater). <br /> <br />Lane's recommended weighted-creep ratios are given for various foundation <br /> <br />materials in Table 3-2. A definition sketch of the variables in Equation <br /> <br />. <br /> <br />3-11 is presented in Figure 3-5 for nonporous liner and cutoffs. <br /> <br />Piping under the structure foundation occurs when the upward seepage <br /> <br />force at the downstream toe of the structure exceeds the submerged weight <br /> <br />. <br /> <br />of material. The soil would be flooded out and the erosion would progress <br /> <br />backwards along the seepage flowline until a "pipe" would be formed, al10wing <br /> <br />rapi d fJ ow under tile foundation and subsequent fai lure of the structure. <br /> <br />. <br /> <br />Cutoff walls, aprons, and drains are generally installed to control <br /> <br />the amount of seepage under the structure and to limit the intensity of <br /> <br />the uplift so that the stability of the structure will not be threatened. <br /> <br />. <br /> <br />The weighted-creep theory does not apply to porous liners such as <br /> <br />riprap. Porous liners with proper filter underneath will prevent the liner <br /> <br />from failure due to seepage force, provided that the submerged weight of <br /> <br />. <br /> <br />the liner is greater than the uplift force. For porous liners, cutoff walls <br /> <br />for the purpose of reducing seepage force are not required. <br /> <br />3.11 Bank Erosion <br /> <br />. <br /> <br />The hanks of alluvial rivers experience varying degrees of flow. Forces <br /> <br />that cause the movement of water through the bank material may be generated <br /> <br />. <br />